An inexpensive Marine Environmental Recorder is described. The instrument system is small, lightweight and of low-power consumption. It is flexible, simple to operate and economical. It can be used remotely in a moored, buoyed or towed instrument system, recording measurements continuously for up to 24 h, or intermittently for 1 min every hour, for a period of up to 60 d. It has been used extensively in the Continuous Plankton Recorder and the Undulating Oceanographic Recorder to measure temperature, depth and occasionally chlorophyll and radiant energy; as a temperature recorder, it has a resolution of 0.1 Co, an uncertainty of measurement of ±0.1 Co and a stability of calibration within ±0.1 Co over a period of several months. With optional additional sensors for pitch, roll, vibration, acceleration and water-flow, the instrument system has been used to measure the performance of underwater towed vehicles and plankton samplers. The Marine Environmental Recorder is being incorporated into an instrument system in a data buoy, for automatically monitoring the marine environment in estuaries around the British Isles.